The present invention pertains generally to nuclear materials and more specifically to devices and methods for determining concentrations of fissionable material, particularly within boreholes.
Prior art techniques of borehole logging for uranium ore have primarily utilized passive gamma ray detection. The disadvantage of the passive detection method is that radioactive decay products from the uranium which emit gamma ray signals are measured rather than measuring the uranium directly. Since these gaseous decay products are oftentimes not in equilibrium with concentrations of uranium ore due to physical migration, assay results can be misleading.
To overcome these disadvantages of the passive technique of detection, active interrogation methods have been developed for direct measurements of concentrations of uranium ore in logging operations. A typical prior art active interrogation device utilizes neutron generation tubes or radioactive .sup.252 Cf as a neutron source for interrogation of the uranium ore. These prior art devices interrogate the uranium ore with neutrons having energies of approximately 500 keV and greater to induce neutrons from the uranium ore for detection by a detector in the probe. Since the induced neutrons have energies of approximately 500 keV to 3 MeV, the .sup.252 Cf interrogation source completely obscures detection of the induced neutrons during irradiation by the interrogation source. Consequently, the interrogation source must be pulsed or modulated to make it possible to measure the time delayed induced neutrons from the uranium ore. Pulsing of the source requires elaborate electrical or electromechanical devices within the probe which are expensive to implement and somewhat unreliable. Moreover, .sup.252 Cf requires extensive shielding for personnel safety and requires special handling techniques in the field.
To overcome these problems of shielding and possible radioactive contamination, DT sources have been used as the interrogation source in some prior art logging probes. However, these interrogation sources are extremely expensive and have a very short operating life, and are consequently, expensive to operate.